In the field of municipal waste incineration it has within the last decades been realised that the flue gas resulting from the incineration must be purified to reduce the amount of noxious components therein.
Mercury is one of the noxious incinerator flue gas components the amount of which it is regarded as essential to reduce, and several measures have been suggested to this effect.
Thus, U.S. Pat. No. 4,273,747 (Rasmussen) discloses reduction of the mercury content in hot incinerator flue gases by quenching said gases by atomizing an aqueous liquid therein in the presence of fly ash suspended in the gas, which quenching causes a cooling from a temperature of at least 200.degree. C. to a temperature below 160.degree. C. The aqueous liquid may be water or an aqueous solution or suspension of an alkaline compound. This method will obviously not be suitable for flue gas from combustion of coal in power plants since the temperature of said gas is substantially below 200.degree. C., typically between 120.degree. and 160.degree. C. Besides, the chemical composition of the major impurities of incinerator flue gas and power plant flue gas are so different that the mercury sorption will be influenced thereby.
European patent application no. 0013567 (Svenska Flaktfabriken) also deals with a process for reducing the mercury content of incinerator flue gas in which the gas is contacted with a solid sorbent consisting of powdered calcium hydroxide and of reaction products from the reaction between calcium hydroxide and gaseous hydrogene chloride. The specific conditions for obtainment of an efficient mercury removal are not disclosed but an efficient mercury removal is described in an example in which an incinerator flue gas is treated apparently having hydrogen chloride as the main pollutant.
In European patent no. 0 253 563 a method for removal of mercury and other noxious compounds from incinerator flue gas is disclosed in which an aqueous liquid containing a basic absorbent is atomized into the flue gas to absorb acidic components from the flue gas and simultaneously to evaporate the water in said aqueous liquid, in which process powdery activated carbon is injected into the flue gas and separated again from said gas together with particulate material formed as a result of chemical reactions and drying of the atomized basic absorbent.
A. S. Stepanov et al. in Prom. Sanit. Ochistka Gazov, 1979 (5)10, abstracted in Chemical Abstracts 92:168288y describe removal of mercury from gas by filtering through a layer of granulated activated charcoal modified by HCl.
European patent no. 0 254 697 describes a method for separating mercury from a water vapor containing gas in which the gas is contacted with a washing liquid in two or several stages in which process the gases are cooled to condense the water vapor in the gas and the gas is washed with a washing liquid containing hydrogen chloride and having a pH of about 3 or below to prevent sulphur dioxide to be dissolved in the washing liquid and to ensure that sufficient halide is present to form a complex compound with mercury. The specification of said European patent application teaches that it is essential that the. pH of the washing liquid is so low that no substantial amount of SO.sub.2 from the gas is dissolved in the washing liquid to form SO.sub.3.sup.2- since sulfite would reduce Hg.sup.2+ to Hg.sup.0, which would evaporate and thereby be re-emitted to the waste gas. Thus, the teaching of this European application is that chloride containing washing liquid for mercury removal must be acidic.
Also U.S. Pat. No. 3,838,190 suggests to remove mercury from gases by means of acidic washing liquids. The washing liquids are sulphuric acid of a concentration of at least 50% containing chlorine and/or hydrogene chloride. The process is described as being suitable for gases arising by the combustion or roasting of sulphide containing ores or gases from electrolysis vessels. Obviously, the process is not suitable for flue gases from power plants.
U.S. Pat. No. 4,729,882 deals with a process for removing mercury from gaseous emissions in which a chlorine containing material is added to the gaseous emissions and the mixture is heated to convert the mercury into mercuric chloride which are removed by scrubbing with wash water and fixed as HgCl.sub.4.sup.2-. The process is described as being suitable for cleaning municipal refuse incinerator emissions. However, when scrubbing of the gas is made by an aqueous solution containing NaCl as Hg binding agent the Hg-removal is substantially below what is achieved by using certain metal complexing agents. The process is a wet scrubbing process thereby having substantial disadvantages when compared with dry or semidry gas purification processes.
Moreover, it should be observed that it is well known to increase the adsorptive effect of activated carbon towards mercury vapor by impregnating the carbon with halogen or inter-halogen compounds as disclosed in U.S. Pat. No. 3,662,523. The process of said U.S. patent, however, involves passing the gas through a fixed bed of impregnated carbon. As described in the above mentioned European patent no. 0,253,563 (incorporated herein by reference) such types of processes are less suited for flue gas treatment.
A discussion of the efficiency of Hg-removal by conventional flue gas purification systems, especially desulfurization methods may be found in a paper by Irene M. Smith: "Trace elements from coal combustion: emissions", IEA Coal Research, London, 1987, pages 54-65. It appears from said paper that in spite of the various systems described above for removing trace elements, especially mercury, from incinerator flue gases, the problem of such removal from coal-fired power plants waste gas still exists.
The flue gas from coal fired power plants differs from the flue gas from municipal incinerator plants in various aspects, especially the pollutants are diluted into a much larger proportion of flue gas; the temperature of said flue gas is lower and the chemical composition of the two types of gases are different.
Thus, the dominating pollutant in incinerator flue gases is often HCl whereas the primary pollutant in gases from coal combustion is SO.sub.2.
Whereas the concern as to Hg pollution of the atmosphere has primarily resulted in the development of various systems for Hg-removal from incinerator flue gas it has within the last couple of years been realized that also the Hg-emissions from power plants etc. represent a substantial risk to the environment.
Since flue gas from coal combustion contains so much sulphur dioxide that a desulphurisation process of the flue gas is necessary, a process for reducing the Hg-content of coal combustion flue gas should preferably be compatible with or preferably incorporated into a gas desulphurisation process.
It has, however, turned out that if the processes used or suggested for incinerator flue gas cleaning in connection with or combined with desulphurisation processes, are tranferred or modified to be used on coal combustion flue gas the results are more or less unpredictable and unreliable as far as Hg removal is concerned, as is further illustrated below.